main/postmarketos-initramfs: unify setup_usb_storage, add pmos.usb-storage= option

Right now we have two similar implementations of setting up a USB mass storage gadget, in init_functions.sh and as separate script for the debug-shell. Given that init_functions.sh is available in debug-shell anyway, unify both in a common setup_usb_storage_configfs() function.

Having this included in the initramfs debug-shell by default (without installing postmarketos-mkinitfs-hook-debug-shell) also simplifies recovery, since you can set up the USB storage for repairing the flashed postmarketOS installation in case of boot failures. The size overhead is minimal, since this has no extra dependencies.

Also, in addition to the pmos.debug-shell cmdline argument, add pmos.usb-storage=<block-device> to set up the USB mass storage gadget by default when going into debug shell. This allows building a special boot image/option that directly exposes the storage for flashing, without having to type in extra debug-shell commands.